Gambogic Acid Efficiently Kills Stem-Like Colorectal Cancer Cells by Upregulating ZFP36 Expression

Gambogic acid (GA), the main active compound of Gamboge hanburyi, has been reported to be a potential novel antitumor drug. Whether GA inhibits putative cancer stem cells (CSCs), which are considered to be the major cause of cancer treatment failure, remains largely unknown. This study investigated whether GA inhibits the CSCs of colorectal cancer (CRC) and its possible mechanisms.

We performed CCK8 and tumor sphere formation assays, percentage analysis of both side population and CD133+CD44+ cells, and the detection of stem cells markers, in order to assess the role of GA in inhibiting the stem celllike features of CRC. An mRNA microarray was performed to identify the downstream gene affected by GA and rescue assays were performed to further clarify whether the downstream gene is involved in the GA induced decrease of the stem cell-like CRC population. CRC cells were engineered with a CSC detector vector encoding GFP and luciferase (Luc) under the control of the Nanog promoter, which were utilized to investigate the effect of GA on putative CSC in human tumor xenograft-bearing mice using in vivo bioluminescence imaging.

Our results showed that GA significantly reduced tumor sphere formation and the percentages of side population and CD133+CD44+ cells, while also decreasing the expression of stemness and EMT-associated markers in CRC cells in vitro. GA killed stem-like CRC cells by upregulating the expression of ZFP36, which is dependent on the inactivation of the EGFR/ ERK signaling pathway. GFP+ cells harboring the PNanog-GFP-T2A-Luc transgene exhibited CSC characteristics. The in vivo results showed that GA significantly inhibited tumor growth in nude mice, accompanied by a remarkable reduction in the putative CSC number, based on whole-body bioluminescence imaging.

These findings suggest that GA significantly inhibits putative CSCs of CRC both in vitro and in vivo by inhibiting the activation of the EGFR/ ERK/ZFP36 signaling pathway and may be an effective drug candidate for anticancer therapies.